kill the systemu guardian thread after the child process has exited.

This fixes a potential thread leak when the parent thread is long running.

[This change](https://github.com/puppetlabs/marionette-collective/blob/master/lib/mcollective/shell.rb#L77) can lead to a thread leak if the parent thread is not short lived like the "agent action" theads.
I use the `Shell` class in a registration plugin which is executed periodically in the long running registration thread. There every usage of the `Shell` class adds another thread to mcollective until it is restarted.